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Middle Jurassic Landscape Evolution of Southwest Laurentia Using Detrital Zircon Geochronology

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Middle Jurassic Landscape Evolution of Southwest Laurentia Using Detrital Zircon Geochronology RESEARCH Middle Jurassic landscape evolution of southwest Laurentia using detrital zircon geochronology Sally L. Potter-McIntyre1, Marisa Boraas2, Keegan DePriest2, and Andres Aslan2 1DEPARTMENT OF GEOLOGY, SOUTHERN ILLINOIS UNIVERSITY, PARKINSON LABORATORY, MAIL CODE 4324, CARBONDALE, ILLINOIS 62902, USA 2PHYSICAL AND ENVIRONMENTAL SCIENCES, COLORADO MESA UNIVERSITY, 1100 NORTH AVENUE, GRAND JUNCTION, COLORADO 81501, USA ABSTRACT The Middle Jurassic Wanakah Formation in western Colorado is a poorly understood unit in terms of depositional environment and abso- lute age of deposition; however, a refined interpretation of the provenance has important implications for understanding the landscape evolution of southwestern Laurentia during Mesozoic rifting of the supercontinent Pangea and the opening of the Gulf of Mexico. This study presents the first U/Pb age dating of detrital zircons from the Middle to Late Jurassic Entrada Sandstone, Wanakah Formation, and Tidwell and Salt Wash Members of the Morrison Formation. Detrital zircon geochronology results show a marked increase in ca. 523 Ma grains (compared to most Mesozoic sediments on the Colorado Plateau) that begins abruptly in the Wanakah Formation and continues into the basal Marker Bed A of the Tidwell Member of the Morrison Formation. U/Pb ages and petrography suggest that the Wanakah Formation was sourced, in large part, from the McClure Mountain syenite on the southwestern flank of the Ancestral Front Range. This abrupt change in provenance occurred due to stream capture and drainage reorganization that input a large amount of water into the basin and caused a shift in depositional environment from the eolian Entrada Sandstone to the hypersaline lake environments of the Wanakah Formation and the Tidwell Member. Additionally, stratigraphic, petrological, and detrital zircon analyses suggest that the contact between the Wanakah Forma- tion and the Tidwell Member of the Morrison Formation is conformable, and the previously interpreted J-5 unconformity is likely not pres- ent in western Colorado. The stream capture and drainage reorganization that created the lake system recorded in the Wanakah Formation and the Tidwell Member likely evolved into the major fluvial system that deposited the Salt Wash Member of the Morrison Formation. The evolution of paleodrainages and provenance are important to understand because they help to constrain landscape evolution across south- western Laurentia, and these insights can help to illuminate the influence of tectonic and sediment controls on depositional environment. LITHOSPHERE; v. 8; no. 2; p. 185–193; GSA Data Repository Item 2016045 | Published online 25 January 2016 doi:10.1130/L467.1 INTRODUCTION widely exposed on the Colorado Plateau (Figs. rado, the type section includes an 18-m-thick 1 and 2). basal limestone member (The Pony Express The Colorado Plateau of the western United The Middle Jurassic Wanakah Formation is Limestone) that is not present north of the west- States in North America was situated in south- exposed throughout western Colorado and ex- ern Black Canyon of the Gunnison (O’Sullivan, west Laurentia during the Mesozoic rifting of tends south into New Mexico; stratigraphically, 1992, 2004; O’Sullivan et al., 2006). In the Black Pangea and the opening of the Gulf of Mexico the unit is above the eolian Entrada Sandstone Canyon of the Gunnison, the Wanakah Forma- (Kocurek and Dott, 1983; Blakey, 1994, 2008; and below the hypersaline lacustrine Tidwell tion contains 3–7-m-thick gypsum lenses, also Blakey and Ranney, 2008; Miall and Blakey, Member in western Colorado (Fig. 1; Peterson, not present at any other exposure (O’Sullivan, 2008). The basin was arid and centered around 1994; O’Sullivan, 2004). A regional uncon- 1992, 2004; O’Sullivan et al., 2006). 30°N. The opening of the Gulf of Mexico pro- formity (known as the J-5) is typically placed The paleogeography is currently recon- vided abundant sediments that were dispersed between the Wanakah Formation and the over- structed largely from depositional environment via Triassic paleodrainages to the NW from the lying Tidwell Member and is delineated by a and provenance studies on the Colorado Pla- Grenville and Ouachita orogens in the central regional sandstone bed called “Marker Bed A” teau (Kocurek and Dott, 1983; Peterson, 1988b; Texas uplift (Lawton and McMillan, 1999; (Fig. 1; Peterson, 1988a, 1994; Turner and Pe- Blakey, 1994; Dickinson and Gehrels, 2003, Dickinson and Lawton, 2001; Mickus et al., terson, 1992, 2004; O’Sullivan, 2004). 2010; DeCelles, 2004; Demko et al., 2004). Re- 2009). In the Early to Middle Jurassic, eolian Controversies exist over Wanakah Forma- cently, advances in U/Pb age dating of detrital deposition dominated this basin; the deposi- tion depositional environmental interpretations zircons have allowed for more refinement in tional environment then shifted to hypersa- (marine or lacustrine) and whether the unit is provenance studies and insight into the maxi- line lacustrine conditions (Peterson, 1994; even present in western Colorado (Tanner, 1970; mum ages of the Mesozoic units (Dickinson O’Sullivan, 2004). Today, these sediments Adler, 1974; Ridgley and Goldhaber, 1983; Pe- and Gehrels, 2003, 2010). This study presents constitute much of the exposures of central terson, 1988a, 1994; Kirkland et al., 1995; Turner new insight into the paleogeographic evolution western North America on the Colorado Pla- and Peterson, 1992; Anderson and Lucas, 1992, of southwest Laurentia during the Middle to teau. The Jurassic shift to a hypersaline deposi- 1994; O’Sullivan, 2004; O’Sullivan et al., 2006; Late Jurassic. Specifically, this new study uses tional environment is evidenced by the so-called Lucas et al., 2006). The Wanakah Formation is detailed analyses of petrography and stratigra- Wanakah Formation and the Tidwell Member lithologically variable and contains several dis- phy coupled with the first detrital zircon geo- of the Morrison Formation—two units that are similar members. For example, in Ouray, Colo- chronology data for the Middle Jurassic rocks LITHOSPHERE© 2016 Geological | Volume Society 8 of| AmericaNumber 2| |For www.gsapubs.org permission to copy, contact [email protected] 185 Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/8/2/185/3051105/185.pdf by guest on 29 September 2021 POTTER-MCINTYRE ET AL. Formation has yielded some absolute age dates from volcanic ash abundant in the formation (interpreted to be deposited from 153 to 145 6 Ma; Kowallis et al., 1998), absolute ages of the Entrada Sandstone (a Middle Jurassic eo- lianite) and the overlying Wanakah Forma tion are not known. Several major unconformities mark the Ju- 5 rassic section on the Colorado Plateau, and this paper focuses on the J-5, which is present as an angular unconformity between the Summerville Formation and the Tidwell Member in southern 4 Utah at Shadscale Mesa (Fig. 2; Gilluly, 1929; 3 Bernier and Chan, 2006). The J-5 unconformity is typically extended into western Colorado and placed in between the Wanakah Formation 2 and the Tidwell Member (O’Sullivan, 2004; O’Sullivan et al., 2006) because the Sum- 1 merville Formation and the Wanakah Forma- tion are interpreted as coeval (Peterson, 1994). However, this interpreted extension of the J-5 unconformity into Colorado is controversial ~1m (O’Sullivan and Pipiringos, 1983; O’Sullivan, 2004). Herein, we investigate the presence of the J-5 unconformity and provenance of the En- Figure 1. Stratigraphy of western Colorado and Escalante Canyon study site. Black dashed lines trada Sandstone, Wanakah Formation, and the show formation boundaries. Contact between 1 and 2 is a scour surface between Entrada Sand- Tidwell Member of the Morrison Formation. stone and Wanakah Formation. Numbers indicate sample locations for detrital zircon analysis. 1—Jebb (Entrada Sandstone); 2—Jeef (sample located above the scour surface and interpreted herein as basal Wanakah Formation); 3—2Jw (Wanakah Formation); 4—3Jw (Wanakah Formation); Purpose of Study 5—1Jmt (Marker Bed A of the Tidwell Member); 6—2Jmt (Tidwell Member). Overlying Salt Wash Member sample location is not shown. Ss—sandstone. The purpose of this study is to refine the re- gional paleogeography and landscape evolution of southwest Laurentia during Middle Jurassic on the Colorado Plateau to (1) provide new in- rek and Dott, 1983; Turner and Fishman, 1991; rifting of Pangea by evaluating the provenance sights on the landscape and drainage evolution Blakey, 2008; Blakey and Ranney, 2008; Miall and depositional environment of the Middle Ju- of the Ancestral Front Range and southwest- and Blakey, 2008). During the Middle to Late rassic section in western Colorado. We utilized ern Laurentia during the opening of the Gulf Jurassic, the depositional environments on the detrital zircon geochronology to determine the of Mexico, and (2) refine the Middle Jurassic northwest part of the Colorado Plateau were stratigraphic position of unconformities above stratigraphy, specifically, the depositional en- tidal environments (e.g., Summerville and Cur- and below the Wanakah Formation and to es- vironments and unconformities present on the tis Formations; Gilluly, 1929; Peterson, 1994; tablish provenance. The specific hypotheses eastern Colorado Plateau. Bernier and Chan, 2006). In the southeast part tested herein are: (1) A distinct and detectable of the basin (in western Colorado and New change in provenance occurred that resulted in a Geological
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